The present invention relates to plasma generators, and more particularly to methods and apparatus for generating a plasma to sputter deposit a film, or layer, of material on workpieces, or substrates, such as semiconductor wafers, in the fabrication of semiconductor devices.
Plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process in a process chamber, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., "sputter", atoms of the material from the target. The sputtered material is then at least partially ionized in the plasma, and deposited on the surface of the semiconductor wafer.
Ionization of the sputtered material is often desirable because nonionized sputtered material has a tendency to travel in straight line paths from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over, causing undesirable cavities in the deposition layer.
To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered in a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma in order to increase the ionization rate of the sputtered material and decrease the formation of unwanted cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density, in the range of 10.sup.11 -10.sup.13 ions/cm.sup.3.
There are several known techniques for exciting a plasma with RF fields, these techniques including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As,shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the primary winding of a transformer. The plasma acts as a single turn secondary winding of the transformer.
Although ionizing the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges. This "center thick" deposition profile is undesirable in many applications where a uniformity of deposition thickness is needed.
In addition, in at least certain types of plasma generators, the plasma density has been observed to vary around the vertical axis of the coil, producing a corresponding variation in the thickness of a deposited film. This variation is such that the film has maximum and minimum values at opposite ends of a line which intersects the coil axis.